The present invention relates to an apparatus for controlling the supply of air to a burner used to recombust diesel particulates trapped in a filter.
In order to prevent air pollution, particulates discharged from diesel engines are usually removed from the exhaust gas by a ceramic filter. At intervals, this diesel particulate filter is subjected to reburning for two purposes, regeneration of the filter and discharging the trapped particulates as a harmless substance. The reburning of the particulates requires a proper temperature and oxygen supply. If the burning temperature is too low, a significant amount of the particulates remains. If the burning temperature is excessively high, the filter itself is burnt.
A burner is frequently used as a heating source for the filter, and one of the atomization type, which atomizes the fuel with a small amount of primary air at a high pressure and burns the particulates with a large supply of secondary air at a low pressure, is most common. The optimum supply rate of the primary air to the burner is substantially proportional to the fuel supply rate, and in order to ensure a constant fuel flow rate, the flow rate of the primary air is usually kept constant. On the other hand, the secondary air flow is at low pressure but must be supplied in a large and controlled amount to ensure the gravimetric air flow rate necessary for burning the particulates. The secondary air is usually supplied by a positive displacement air pump, which type of pump ensures a constant volumetric air flow rate if the rotating speed is held constant. On the other hand, the required flow rate is sensitive to variations of the atmospheric pressure and ambient temperature, as well as in the pressure of the exhaust gas. It is therefore required that, with the use of a positive displacement air pump, any variation in the gravimetric flow rate be corrected without sacrificing the most significant advantage of this type of pump, namely, a high air discharge rate.
Common positive displacement air pumps have characteristics as shown in FIGS. 1 to 3. FIG. 1 shows an example of the volumetric flow rate vs. discharge pressure characteristics. From FIG. 1, it can be seen that, by reducing the cross-sectional area of an air line on the discharge side, the volumetric flow rate of air is decreased whereas its discharge pressure is increased. FIG. 2 shows an example of the gravimetric flow rate vs. discharge pressure characteristics for different altitudes at which the air pump is used; the results at a low altitude are indicated by the solid line whereas those at a high altitude are represented by the dashed line. As can be seen from this Figure, in order to obtain the same gravimetric air flow, the discharge pressure at high altitudes must be made lower than at low altitudes by increasing the cross-sectional area of the air line. Even if the altitude is the same, the gravimetric flow rate from the positive displacement air pump varies (as shown by the two dashed lines in FIG. 3) depending upon fluctuations in the pump performance and the atmospheric pressure.
An example of a conventional particulate filter system supplying secondary air with a positive displacement pump having the characteristics shown above is illustrated in FIG. 4. A diesel engine generally indicated at 1 includes a turbocharger 2 and a filter 5 in an exhaust line 3 at a point downstream of the turbocharger 2. The exhaust gas is discharged through a muffler 200 positioned downstream of the filter 5. A burner 4 is provided in the exhaust line 3 at a point upstream of the filter 5. The burner has an ignition unit using an ignition coil 6. The burner atomizes the fuel from a fuel pump 8 with primary air from a pump 7 whose flow rate is adjusted by a pressure regulating valve 201. At the same time, the burner uses secondary air from a pump 9 to produce a hot gas having a predetermined excess air ratio. Using the excess oxygen, the burner burns the particulates trapped in the filter 5. The cross-sectional area of a secondary air line 10 is adjusted by the operation of a flow control valve 11, and a vacuum chamber for actuating the switching operation of this valve is connected to a vacuum pump (negative pressure source) 12 via a vacuum regulating valve 13 and a solenoid valve 14.
With the system shown in FIG. 4, it is necessary that the flow of exhaust gas have no adverse effects on the regeneration of the particulate filter. In order to meet this requirement, as shown in FIG. 4, the exhaust line 3 is provided with a bypass 202 that is connected to the line 3 at two points, one upstream and the other downstream of the line. A valve switch 210 is positioned at the upstream junction between the exhaust line 3 and bypass 202. The valve switch 210 is driven by a link mechanism connected to a diaphragm 203 which further communicates with the vacuum pump 12. A solenoid valve 204 is provided between the diaphragm 203 and vacuum pump 12. The solenoid valve 204 is composed of a plunger 205, a coil 206 and a spring 207. When the coil 206 is energized, the plunger 205 is attracted toward the coil 206, thereby opening the valve 204. Then, the negative pressure in the vacuum pump 12 acts on the diaphragm 203 and the valve switch changes its position from a to b so as to close the exhaust line 3. As a result, the exhaust gas from the engine 1 is guided to the muffler 200 through the bypass 202. Accordingly, the exhaust gas from the engine 1 has no effect on the combustion in the burner 20. In FIG. 4, reference numerals 17 and 18 indicate a fuel regulating valve and a pressure regulating valve, respectively. Reference numeral 15 indicates a controller for controlling the ignition coil 6, air pumps 7 and 9, solenoid valve 14 and the fuel regulating valve 17. Reference numeral 16 refers to an atmospheric pressure sensor.
When the filter 5 is overloaded with particulates from the engine 1, the controller 15 detects with the senosr 19 that the pressure in the exhaust line at a point upstream of the filter 5 has exceeded a preset value, and upon detection of this fact, the controller initiates reburning of the particulates in the filter. If the engine is running at a high altitude where low atmospheric pressure is prevalent, an input signal from the atmospheric pressure sensor 16 causes the controller 15 to produce the necessary output to the solenoid valve 14 so as to increase the cross-sectional area of the secondary air line to a level which is greater than the reference level by a given amount. This produces an increase in the volumetric air flow rate that compensates for the decrease in the gravimetric flow rate due to a drop in the density of air. However, this system simply controls the change in the level of atmospheric pressure by the flow control valve 11 which relies on a diaphragm that receives a constant negative pressure. A significant problem with this diaphragm system is its inability to control the flow of secondary air with high accuracy, and this is particularly so if the characteristics of the secondary air pump vary.